This invention relates to a method for producing carbon and graphite fibers. More particularly it relates to a method for producing carbon and graphite fibers from polyacrylonitrile fibers.
The need for composite materials that have high temperature strength, stiffness and toughness is well known. For example, reinforced plastics have found use in widely divergent applications such as golf club shafts, rocket tubes and heat exchangers.
One method of manufacturing such composites involves the combination of resins with carbon and graphite fibers. In addition to possessing high temperature strength, stiffness and toughness, these composites are also light weight. Frequently carbon and graphite fibers used in these composites are formed from polyacrylonitrile fibers. However, carbonization of said fibers is inherently difficult due to the occurrence of a sharp exotherm at 250.degree. C-350.degree. C. It is believed that the exotherm results when polyacrylonitrile changes from a linear to a cyclic structure. Fibers obtained when the exotherm occurs are often brittle and fused together. Embrittlement is also accompanied by shrinkage of the fiber dimensions (e.g. about 30 percent reduction from original fiber size).
Various attempts have been made to produce carbon and graphite, hereinafter referred to as carbonaceous, fibers. For example, U.S. Pat. No. 3,412,062 discloses the production of carbonaceous fibers from polyacrylonitrile fibers by heating them from 25.degree. C to 1000.degree. C at a rate of 15.degree. C per hour in a vacuum while they are under tension. The resultant carbon fibers may then be graphitized by heating them at 2,500.degree. C for 1 hour in an argon atmosphere. Other similar methods for the production of carbonaceous fibers from polyacrylonitrile are disclosed in U.S. Pat. Nos. 3,556,729 and 3,607,059. Each of these patents require the use of tension and slow initial heating in order to prevent the polyacrylonitrile fibers from becoming brittle or fusing together. In order to maintain the constant tension, special equipment is required. Such equipment is expensive and, consequently, designed to be utilized most economically in large scale continuous production. The resulting high cost seriously limits the utilization of carbonaceous fibers.
Another method for the production of carbonaceous fibers is disclosed in U.S. Pat. No. 3,592,595. In this method polyacrylonitrile fibers are treated with a solvent solution of a Lewis acid comprising a complex of a metal salt (e.g. tin chlorides, iron chlorides, etc.), and dimethyl formamide to form a stabilized product that can in turn be converted to carbonaceous fibers by heating it to at least 1,000.degree. C. The metal salts used in this method are expensive. Moreover, large quantities of solvent are necessary in order to form the complex and prepare the solution thereby further increasing the cost of this method and introducing the additional concerns of health, safety and the ecology.
The present invention solves these and other disadvantages attendant with the prior art thereby producing relatively inexpensive carbonaceous fibers. Moreover, the present invention allows preparation of the carbonaceous fibers to be interrupted at any stage of the process without affecting either the intermediate or final products of the process. Furthermore, the intermediate products can be stored for long periods of time (e.g. several months or more) without affecting the final product.